Let’s deal with WeChat — the second most popular messenger in the world

… there is no «read» status [in SMS]

AFAIK WeChat messages also do not show their «read» status to the user, unlike e.g. Telegram where one tick means that message had been received by the server and would be delivered to the recipient as soon as she gets online, two ticks mean that message had been read. WeChat messages do not have ticks.

I would also like to inform you that there is no end-to-end encryption between users. Messages are encrypted only by a symmetric key, the server decrypts them and re-encrypts them with a symmetric key of the recipient and sends them to the recipient.

Like there'd been any doubt they're doing that.